The invention relates to a method of constructing underground galleries and finds particular application in the construction of tunnels which may be considered as a series of galleries joined end-to-end.
Throughout the specification the term "gallery" is used to describe the invention and such term is intended to include isolated galleries and chambers as well as tunnels.
The construction of tunnels has traditionally raised problems due to the large forces exerted by the earth or ground medium surrounding the tunnel. The construction of large diameter tunnels, such as are suitable for accommodating subways or roads, has proved particularly problematical due to the relatively large structural dimensions involved. On the other hand, the laying of relatively small diameter pipes is not so difficult since it is less easy for the overlying earth to crush the pipe. The present invention is primarily concerned with the construction of large diameter galleries such as, for example, subway tunnels.
Traditionally underground galleries and similar structures have been formed by one of two methods.
The first method involves boring below the earths surface and shoring up as the gallery or tunnel progresses.
The second method is the so-called "cut and cover" method whereby a pit or trench is excavated and the gallery or tunnel is assembled or built on the floor of the pit or trench. Thereafter the the trench is filled to cover the gallery or tunnel. In such a case the gallery or tunnel itself supports the entire load of the infilled earth material.
The present invention represents an improvement in the cut and cover method and makes it possible substantially to reduce the thickness of the gallery shell, thereby effecting a saving in materials. This is made possible by the fact that the applied backfill together with the gallery acts to support the total load of backfill, buildings, traffic, etc, thereby, removing much of the stress which would otherwise be applied directly to the gallery shell. Put another way, structures for galleries constructed by the "cut and cover method" must withstand by themselves all stresses originated by the total backfill weight. Consequently, those structures use large quantities of structural materials which substantially increases the cost of construction. In addition the greater consumption of materials complicates the use of precast units because of excessive weight or the need of a great number of joints. The present invention makes possible a reduction in the amount of structural materials utilised in galleries since the backfill becomes a structural element and the stresses originated by the total backfill weight are withstood jointly by the structure of the gallery and the backfill.